Agriculture Reference
In-Depth Information
Discussion and Perspectives
basic metabolism for maintenance even
though the measurement of the energy cost of
physical activity remains difficult. The use of
a simple behavioural quantification and an
energy transformation is consistent with data
from the literature but requires additional
validation.
Previously most authors have considered
growth performance from an energetic stand-
point. They usually took for granted that feed
intake was the consequence of growth, as
the means to achieve their growing potential.
However, many factors (i.e
.
nutritional and
environmental ones) are known to be involved
in the regulation of feed intake, making it
difficult to predict feed intake precisely by
considering only nutritional requirements.
We chose to consider growth as a consequence
of energy intake. To regulate feed intake, IN-
AVI is based on the thermostat concept,
which computes the energy balance between
a reference model in 'optimal' conditions
and a simulation model in 'non-optimal'
ones. This concept stresses that thermal bal-
ance is a key issue for the animal, and it
could be considered as the homeostatic regu-
lating mechanism, since the animal will try
to adapt its heat production according to its
capacity to evacuate it on a short-term scale
(less than
1
h). However, since energy bal-
ance is not the only key to the regulation of
feed intake, INAVI also takes into account
regulations based on feed characteristics
(composition, size of particles). It could be
asked what the threshold is above which the
bird would be unable to maintain its internal
temperature, and what crisis mechanisms
should be added to the model.
Despite the underlying complex biological
mechanisms (regulations loops, energy bal-
ance, metabolic efficiencies, etc.) included in
the model, INAVI has a simplified architec-
ture. It includes a low number of parameters
in order to remain both user-friendly and bio-
logically relevant. Our approach is the result
of conflicting issues: a mechanistic process is
required to model broiler growth, but models
have to be simple and useful tools for mul-
tiple stakeholders, who are not always used to
the underlying concepts. This simplification
led us to take some shortcuts, for which we
are fully open to criticism, such as a constant
basic efficiency of deposition (
Ed
) (even though
it is modified by response laws) and a global
energetic value of deposition (
Ved
). INAVI
also led to us renewing the concept of main-
tenance by separating physical activity from
Taking into account body composition
in the representation of weight gain
In INAVI, growth is considered as a conse-
quence of the global use of ME by the ani-
mal (i.e. the difference between ME intake -
heat production) with the introduction of
two parameters describing the use effi-
ciency of ME (
Ed
) and the energetic value of
deposition (
Ved
), which converts energy
into body weight gain. Total growth can also
be seen as the sum of the growth of several
compartments (protein, fat, ash, water) as
proposed by Emmans (1995). Both ap-
proaches (intake or growth as driving forces)
could be assumed to be relevant, specific-
ally under a push-pull representation.
A precise description of body compos-
ition, especially protein and ash weight
therefore could be used for the prediction
of nitrogen and phosphorus excretion by a
mass balance approach between ingestion
(feed intake) and retention (deposition).
Nutritional strategies could be thus evalu-
ated both on economical (growth, feed
intake) and environmental (nitrogen and
phosphorus excretions) bases. Moreover,
body composition could also be used in
order to evaluate the quality of carcasses,
mainly by considering carcass fattening
and the weight of physical parts with an
economic interest such as breast or drum-
stick (Danisman and Gous, 2011).
Future improvements concerning
growth and intake regulation
Temperature effects
The perceived temperature described previ-
ously takes into account several environmental
